A compound or its salts that inhibit the activity of the polypeptide or receptor of the present invention and the antibody of the present invention as well as the antisense DNA of the present invention are useful as, e.g., gastric acid secretion inhibitors, mucosa protectants, mineral absorption promoters, etc., which are less toxic and safe, and can be used as, e.g., agents for preventing/treating upper digestive tract disorders, antibacterial agents to Helicobacter pylori, etc. A compound or its salts that promote the activity of the polypeptide or receptor of the present invention, the polypeptide or receptor of the present invention and the DNA of the present invention can be used as, e.g., gastric acid secretion promoters and can be used as, e.g., agents for preventing/treating dyspepsia, bone metabolism disorders, anemia, etc. A compound or its salts that promote the activity of the polypeptide or receptor of the present invention, the polypeptide of the present invention, etc. can also be used as test agents for gastric secretory function. Furthermore, the polypeptide or receptor of the present invention and the DNA of the present invention are also useful for screening the agents for the prevention/treatment described above.